Hey new to the forums here and thought I'd put my $.02 in.
What if now this is a pretty good what if....Anakin got that gash from some past battle that him and Obi were in while fighting in the war. Then the fire that burns him at the end in ROTS just simply reopens this gash. Might it be that simple? 😉
Anakin's clothes and hair made him burn. Skin doesn't burn like clothing does. There needs to be something on the skin to cause such burning.
His gash is caused by fire. Fire is random. It just decided to burn heavily on that part of his head. Makes sense.
As for the "helpless" thing, I didn't think so. I never got the impression that he was reaching out to Obi-Wan. I got the impression he was in a lot of pain, but it fuelled his anger and hate, the bits where he is clawing up the bank gave me the impression he was still trying to fight, he was trying to get back to Obi-Wan, like the killer in a horror movie. Even under such pain, he was still trying to attack. I don't think for one second he had a look of "help me". His look was more like "I'm gonna get you! Come here!".
It was a brilliant image, of Anakin becoming totally inhuman and machine like. He was like an insect with no legs, like a machine. I thought it was great.